imgact_elf: procstat groups: Restore sending the effective GID

Fixes 'procstat -s' run on a core file when the number of groups in
effect at the moment of core dump exceeds KI_NGROUPS (16).

Reviewed by: kib
Fixes: be1f7435ef218b1d ("kern: start tracking cr_gid outside of cr_groups[]")
MFC after: 9 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52258

(cherry picked from commit 5568b4441dff078733a76835312a6ce1ccb50a17)